If you book via the UK website of one of the big companies (Hertz or Alamo are recommended) then you get everything you need (insurance, excess waiver) as standard. Dont bother with any of the additional services like enhanced breakdown or other insurances. Be careful with companies like Enterprise that cater more to the domestic market where insurance may not be included (domestic drivers use their own insurance for car rentals).
Thanks for the advice!

Hertz website doesn't seem to be working, and Alamo's only pickup location in LA is LAX, so I can't use them. Need a downtown collection. What about Avis?